BY HOLLICE F. STONE, M.S.,P.E.
For many years, Americans have known that terrorist attacks on symbols of our government were a definite risk … abroad. With the attacks on the Pentagon and the World Trade Center on September 11, 2001; the war in Iraq; and other recent vehicle bombing events, it has now once again been brought forcefully home to us that the potential for domestic terrorist attacks continues to exist.
EFFECTS OF BLAST
The unique nature of explosive attacks and their effect on buildings and occupants make it important that first responders understand some key issues relating to blast. Discussed here are the relevance of preparing for explosive attacks, the fundamentals of explosions, explosion effects on buildings and their occupants, and recommendations for firefighters responding to bomb threats and explosive events.
RELEVANCE
You may ask why we should be concerned about car and truck bombs when the magnitude of the World Trade Center and Pentagon attacks was so much greater than previous attacks? The answer is that the likelihood of a repeat car or truck bombing event is still high. This conclusion is based on several factors. First, a review of terrorist events over the years shows that car and truck bombings have the highest level of recurrence. Second, the level of planning and number of years that went into the execution of the September 11 attacks is extensive, and the number of groups with the ability to make this happen is small. On the other hand, it is relatively easy and inexpensive to create and deliver a vehicle bomb.
BLAST EFFECTS: THE BASICS
An explosion is an extremely rapid release of energy in the form of light, heat, sound, and a shock wave. The shock wave consists of highly compressed air traveling outward, in all directions, from the source at supersonic velocities. As the shock wave expands, pressures decrease. When it meets a surface that is in the line of sight of the explosion, it is reflected and amplified by a factor of up to 13.
These events are extremely fast; are measured in milliseconds instead of seconds; and are characterized by a large, rapidly increasing, positive pressure followed by a negative pressure, which creates a suction effect. This suction can pick up and carry flying debris in the vicinity of the detonation.
The shock wave causes the initial damage and injuries. The pressures exerted on anything in its path (be it a building or a body) are enormous and can be thousands of times greater than those created by the strongest hurricanes.
BUILDING TYPE RESISTANCE TO BLAST EFFECTS
Many factors contribute to how a building will respond to an explosive event. As noted above, the size of the bomb and the distance from the bomb contribute to the amount of pressure exerted on the structure. The next item of importance is the type of building construction. Each building type tends to exhibit a different response to the same pressures and durations.
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First, it is important to remember that the vast majority of buildings were not built with explosive loading in mind. Therefore, just because a building does not respond well to a bomb does not mean that the building was poorly designed. Blast loading is an extremely abnormal event and stresses buildings in ways never anticipated during the original design and construction. Buildings are generally designed to hold up gravity (downward) loads. In earthquake regions, they are designed to withstand side, or lateral, forces created by ground movements. Standard buildings are not designed to withstand large, aboveground shock waves of the magnitude associated with explosions. With this said, Table 1 shows the relative blast resistance of different conventional construction types.
BUILDING DAMAGE
Explosion damage patterns can be categorized into three major categories: nonstructural damage, structural damage, and collapse.
Nonstructural damage. This category refers to the damage or destruction of elements that do not support the structure’s vertical load. These include windows; infill walls (masonry or concrete installed between beams and columns); exterior cladding; parapets; exterior signs; ceilings and ceiling-mounted fixtures; mechanical, electrical, and electrical piping and equipment; interior finishes; and furnishings.
Much nonstructural damage will be obvious at first glance. Windows may be blown out, parapets may be collapsed, and interior furnishings may be thrown about. However, some of the nonstructural damage may not be immediately obvious. An example of this includes exterior precast panels. Connections of these large, concrete panels are designed mainly to hold up the weight of the panels and are not designed to resist the large pressures (both inward and outward) created by an explosion. These connections will be stressed beyond their original design intent and should be considered suspect until they have been evaluated.
Special note should be taken of windows because they are very fragile elements and do not respond well to explosive loading. Window breakage (and possible glass laceration injuries) can be expected for many blocks surrounding an explosion.
Structural damage. Structural damage refers to failure of major structural elements (columns, bearing walls, girders, and so on) without causing additional failure of surrounding structural elements. Generally, structural damage will occur at the exterior vertical elements for an exterior vehicle weapon attack or in the immediate vicinity of an interior weapon attack (i.e., the 1993 World Trade Center attack).
Columns and beams may be damaged or have completely failed. Damaged columns will be severely out of plumb, bowed, twisted, or partially sheared. Floor slabs may be blown away or damaged to the point where they no longer can support vertical loads. Damage to floor slabs may include rebar being stripped away with the concrete remaining in place, bowing of the slab so that it resembles a cable draped between the supports, and complete or partial shearing of the connections to the supports. For low-rise buildings, the roof may also experience structural damage. This can include failure downward into the occupied space or upward (because of the suction phase of the explosion), in which case it will be thrown clear of the building, possibly further damaging surrounding buildings or trapping and injuring people.
Collapse. Collapse represents the greatest hazard potential for building occupants and rescuers. The collapse may be caused by the shock wave itself, or it may be progressive and the result of damaged primary structural members. Only a portion of the building may collapse, as in the Alfred P. Murrah Building in Oklahoma City, Oklahoma, or it may be a total collapse as in the 1998 bombing of the Ufundi House, the low-rise structure next to the U.S. Embassy in Nairobi.
Collapse caused by the air-blast wave occurs during the explosion and may include columns and beams that fail because of the magnitude of the pressures exerted on them. Failure often occurs in connection points, which often are the weakest links in the construction. Collapse caused by the blast wave is generally localized in the immediate region of the explosion, because pressures decrease rapidly as the distance from the bomb increases.
A progressive collapse involves structural elements that did not fail as an immediate result of the explosion but failed because of the loss of their supporting members. Progressive collapse is akin to what happens to a house of cards when one of the lower cards is removed. Buildings that have large column spacing, large floor-to-floor heights, thin floor slabs, transfer girders (horizontal elements that funnel the vertical load from many columns above into fewer columns below), and long slender columns may be susceptible to progressive collapse.
Progressive collapse may occur within seconds of the explosion or hours later. The time difference depends primarily on the redundancy (multiple ways for loads to get from the upper portions of the building to the foundations) in the original building design. Generally, buildings designed to resist higher earthquake loads (i.e., in Los Angeles and San Francisco) tend to have greater redundancy than buildings not designed for earthquake loading. The more redundancy in the building, the greater the time span between the explosion and potential collapse. Highly redundant buildings may not collapse at all.
The collapse at the Alfred P. Murrah Building is an example of a structure that experienced immediate progressive collapse. When its vertical elements were removed, floor slabs were stripped away from columns and a transfer girder failed.
Structurally damaged buildings may experience delayed progressive collapse because of the eventual failure of the damaged structural elements. Warning signs include columns that have had slabs stripped away from one or more floor levels, buildings that have completely lost one or more columns, and walls that have pulled away from the floors and roof. The collapse of the Twin Towers of the World Trade Center is an example of buildings that experienced a delayed progressive collapse.
AREA DAMAGE PATTERNS
Firefighters responding to an explosion should have an idea of what type of damage patterns they may find on arrival. Generally, collapse will occur closest to a bomb; structural damage will occur at a moderate range; and nonstructural damage (especially window breakage) will extend for a great distance from the explosion.
However, as noted above, different construction types have varying levels of resistance to explosions. This means that structural damage or collapse of relatively weaker buildings may occur beyond the rule-of-thumb distances. This happened in the Oklahoma City bombing where many of the buildings that collapsed were farther from the detonation than buildings that did not collapse. Because of this, rescuers must evaluate buildings in a large perimeter around the actual explosion.
INJURY TYPES
As with structural damage, human injury may be caused by the air blast itself or by collapse and flying debris.
Injuries from direct air-blast effects may include fatalities, lung collapse, and eardrum rupture. Injuries from structural and nonstructural damage include crushing, blunt trauma from flying debris, impact from being thrown against solid objects, and glass lacerations from flying glass.
The greatest number of fatalities will be found close to the point of detonation; victims will have suffered from direct air-blast effects or crushing in a collapse. As the distance from the explosion increases, so, too, will the likelihood of finding live victims. Glass laceration injuries likely would be the most prevalent injury outside the building’s collapse zones and may occur at significantly large distances from the site.
DANGERS TO RESCUERS
Each damage category discussed above presents unique dangers to rescuers based on the stability of the remaining structure. The following section discusses some of the risks associated with the categories.
Nonstructural damage. Nonstructural damage can occur at the building’s perimeter and interior and at large distances from the building. On the interior, debris will block access routes and pose continuing overhead fall hazards. Reports from the rescue efforts at the African Embassy bombings (1998) noted that debris from nonstructural elements continued to fall during the recovery efforts. Additionally, it was reported that debris from nonstructural damage hindered evacuation in some locations.
On the exterior, rescuers must be aware of severely damaged nonstructural elements that have not yet released from their original location. Of special note are precast panels, windows and glass, signs, and parapets.
Structural damage. Along with partially collapsed buildings, structurally damaged buildings can pose the greatest danger to rescuers. The building is still basically intact, but the weight is now carried by compromised structural elements. Rescue operations may impact damaged members, vibrations from rescue equipment may increase damage, or shifting of the debris pile may redistribute the load beyond the capacities of the remaining structure.
In addition to the potential for further collapse, dangers to rescuers include overhead falling hazards and shifting of debris.
Collapse. For buildings that collapse from the actual air-blast pressures or that progressively collapse, the dangers to rescuers will come from shifting of the debris pile during rescue operations, falling hazards, and secondary collapse hazards from portions of the building adjacent to the collapse zone.
Buildings that experience delayed progressive collapse pose the greatest danger to rescuers. Because of this, it is important that a rescue-trained engineer evaluate the building and that a stabilization plan be implemented before executing extended extrication operations.
RECOMMENDATIONS FOR RESCUERS
Bomb Threat Response
During a bomb threat, rescue personnel may be involved in evacuation efforts or will stage to be on hand if there is an explosion and subsequent rescue efforts are needed. All involved personnel should be aware of the unfolding situation, gather as much information as possible, and use common sense at all times. The following are some recommendations for emergency responders to a bomb threat:
Determine the actual threat. If a suspicious vehicle is at the perimeter of a building, determine the location of the vehicle and ensure that evacuation and staging locations are far from the suspect vehicle.
Keep your distance from the primary target building. It is important to protect personnel from immediate air-blast and collapse effects. The federal government has developed criteria for determining safe evacuation distances. They are presented in the Terrorist Threat Bomb Standoff Card, which may be ordered at http://bookstore.gpo.gov. This card shows safe evacuation distances for different sized weapons, based on the delivery vehicle. The distances in this card are recommendations; good judgment must still be used when applying the information.
Be aware of your surroundings at the staging location. Stay away from unreinforced buildings, parapets, overhead signs, overhead power lines, transformers, and large glass curtain walls.
Wear personal protective equipment. Rec-ognize that glass and other lightweight finishes will break or release from buildings dozens of blocks away from the bomb. Wearing protective gear will reduce the potential for injuries.
Explosion Response
After an explosion, firefighters must balance care to protect rescuers with timely response to victims. The following are some recommendations that may reduce the risk for emergency personnel responding to an incident.
- Assess structures in all directions from the blast. Hazards and victims may be present in the target building as well as in nearby buildings. Hazards at nearby buildings may include structural damage (and, therefore, high potential for instability) or broken and falling windows, cladding, parapets, and signs.
- Create a “safe” zone. Keep personnel not involved in the active portion of the rescue operation far enough away from compromised buildings to prevent unnecessary injuries from secondary collapse or the release of falling hazards. The location of the safe zone should be based on the results of the assessment of the target and nearby buildings.
- Access the building from the point as far as possible from the explosion site. These areas have the highest potential for structural stability.
- Limit the number of rescuers and duration of time in the building until the building has been stabilized.
- Have a rescue-trained engineer evaluate the building before beginning long-term or mechanized extrication operations.
- Implement building stabilization measures as soon as possible.
- Beware of warning signs and hazards, including cracked, bowed, or twisted walls and columns; columns that have lost surrounding slabs; slabs that have lost support or are cracked and bowed; exterior elements that may come loose during the explosions; and interior finishes that may continue to “rain down” on rescuers.
Bomb blasts are low-probability/high-consequence events that present unique challenges and risks for emergency responders. By maintaining a heightened awareness of potential hazards, keeping command and staging areas outside the potential collapse zones, and using engineering consultation and building-stabilization techniques, responding personnel can limit risk to rescuers and effect more efficient response operations.
HOLLICE F. STONE, M.S., P.E., is a lead engineer for Hinman Consulting Engineers, Inc., a firm specializing in designing buildings to resist the effects of explosive terrorist attacks. With Hinman, Stone has developed blast-resistant designs for the U.S. Department of State, U.S. Department of Defense, U.S. Department of Justice, and General Services Administration. In conjunction with the San Jose (CA) Fire Department, she has partnered with the General Services Administration, the Department of Homeland Security, and the Protective Glazing Council to test the effects of blast-resistant window systems on firefighter emergency access and egress. She is a member of the California Task Force 3 FEMA Urban Search and Rescue Team and is trained as a logistics manager and structural specialist, providing post-incident support to rescue/recovery efforts after disasters caused by explosions, earthquakes, and floods. In this capacity, she was deployed to New York in the aftermath of the September 11, 2001, collapse of the World Trade Center.
